In the area of nuclear magnetic resonance, the need for apparatus equally sensitive to non-adjacent spectral regions is encountered in several contexts. One common example occurs where a sample is irradiated at some (high) frequency for one purpose while the same sample is concurrently irradiated at another (low) frequency for some other purpose. This is typical of a decoupling experiment wherein, for example, C.sup.13 -hydrogen couplings are disturbed while separately exciting the carbon 13 resonance concurrently.
One variation of such a double-tuned arrangement is the need for excitation and observation of chemically distinct samples where one such sample is a control employed for instrumental purposes such as the establishment of a field frequency lock, while the second sample is under study. One example of this arrangement is to be found in U.S. Pat. No. 3,434,043, commonly assigned. Another similar circumstance is the desire to concurrently excite selected different nuclei for acquisition of corresponding spectral response.
A double-tuned circuit often utilizes a single inductor common to two resonant circuits. Each sub-circuit in such an arrangement is separately tuned and impedance matched to its respective rf source (or sink). In one class of double-tuned arrangement, it is necessary for an isolation element to be inserted between high frequency and low frequency sub-circuits, if it is required to achieve concurrent excitation at the respective frequencies. If it is sufficient to apply rf energy at different frequencies non-concurrently, there need be no isolation element. This is often the case where separate observations are to be made and it is desirable to avoid a re-tuning procedure or disturbance of the subject/sample. Moreover, the necessity for plural, isolated ports may be less for resonance observation than for resonance excitation. This is the case where, for example, two resonances of substantially different magnitude, a single port matched to the weaker resonance will attenuate the stronger resonance. If such attenuation of one resonance is not desired, it may be preferable to have the benefit of multiple, isolated ports.
Double-tuned circuits are known which employ a transmission line of length .lambda./4 (at the high frequency) to provide such isolation. For an example of such an arrangement, see Stoll, Vega, and Vaughan, Rev. Sci. Inst., v. 48, pp. 800-803 (1977).
A combination of 1/2 .lambda. transmission lines has been used to provide a double-tuned arrangement for frequencies in a ratio of a power of two to form a series arrangement of separate inductors at one frequency and a parallel arrangement of the same inductors at another frequency. This work is discussed in U.S. Pat. No. 5,038,105, commonly assigned.
Inductive elements in rf probe circuits are known to include "split inductors" such as taught in the work of Alderman and Grant, J. Mag. Res., v. 36, pp. 447-451 (1979) and Cook and Lowe, J. Mag. Res., v. 49, p. 346 (1982).
Balanced circuits exhibiting electrical symmetry are also known for the purpose of supporting double-tuned apparatus. Such circuits exhibit, among other properties, the virtue that a symmetry plane (or other surface) is defined which has a property of electrical neutrality, which is to say, a virtual ground.
An example of a balanced double-tuned circuit with split inductors and capacitances for NMR observe coils is to be found in U.S. Pat. No. 4,833,412. A double-tuned balanced circuit using lumped elements for a birdcage geometry is described in U.S. Pat. No. 4,916,418 commonly assigned herewith. Yet another double tuned coil, well suited to localized spectral distributions and imaging is described in U.S. Pat. No. 5,057,778, commonly assigned herewith.
The present invention describes a saddle coil exhibiting multiple resonant behavior. Saddle coils define a sample volume of generally cylindrical symmetry and impose a radially directed magnetic field on that sample volume.